This relates to computer audio systems, and more particularly, to a method and apparatus for providing audio in different formats simultaneously using the same stereo jack.
Competitive pressure and customer demand in the computer industry is currently forcing computer manufacturers to lower cost, reduce size, and increase performance of their products. The physical size of computer systems has decreased to the point where a system that fits on a user's desk or lap has greater capability than systems that used to fill an entire room. Along with demand for ever increasing processing speeds and higher resolution graphics displays, there is increasing demand for high-fidelity audio systems, such as 5 channel surround sound, in today's computer systems. Physical space limitations in today's computer systems often force trade-offs between capability and size.
Additionally, computer software applications that use digitized speech, audio, image, and video, have data storage and transmission capacity requirements that have grown and continue to grow at remarkable rates. In response to these requirements, data compression techniques are used to eliminate redundant and unusable information in the data while maintaining acceptable fidelity of the signal, thereby conserving storage space and transmission bandwidth. Several different compression techniques are commonly available for different types of data including speech, audio, image, and video data. Once the data is compressed, it is typically transmitted to a storage device for later retrieval, or to a device controller for output to a particular device such as a display monitor and a set of speakers. The compressed data is decompressed before being output to the device controller.
To preserve digital signal quality, it is desirable to avoid converting between analog and digital and transmit the compressed data entirely in the digital format. Thus, different devices must be able to communicate with one another within the digital domain. Transmission messages are a combination of digital signals and control messages that are formatted depending on the transmission standard being used. For example, one transmission interface standard that is commonly used to transmit digital audio data is the Sony-Philips Digital Interface Format (SPDIF), standardized by the International Electrotechnical Commission, Geneva, with reference number IEC958-1989.
SPDIF messages are divided into blocks of 192 frames, each frame comprising a number of sub-frames of 32 bits. The number of sub-frames that are used depends on the number of channels that are transmitted. For example, a CD player uses channels A and B (left/right) and therefore each frame contains two subframes. Each sub-frame includes a 20-bit field that carries audio signal information, and various control and information bit fields. SPDIF allows two types of data over the interface, compressed Dolby AC-3 (which includes 5.1 channels of surround-sound audio) and linear pulse code modulated (LPCM) (which includes 2 channels of CD-quality stereo audio). One of the bit fields is a channel status bit that indicates whether the channel format is AC-3 or LPCM. These formats therefore cannot be output simultaneously using SPDIF.
There are often situations where it is desirable to output both AC-3 and LPCM audio at the same time. For example, digital versatile disk (DVD) players are capable of outputting Dolby surround sound audio which uses AC-3 compressed format. Other audio components, such as CD players and voice synthesizers output LPCM audio. In many instances, there is a need to provide DVD audio while at the same time providing an audible message such as a low battery warning or e-mail notification.
Although it is possible to include a connector for each format, there may not be an adequate amount of space available for an additional connector on the backplate of the audio card. Further, one goal in designing computer systems is to reduce the number of components to the maximum extent possible for space and weight saving. Thus, an audio card capable of providing both AC-3 and LPCM audio signals from the same connector is desired.